Chilled water plants are employed to provide cooling for building comfort loads and for industrial process loads and are a major user of electrical power. Chilled water plants generally employ multiple chillers, and some chillers employ multiple compressors. This permits equipment to be staged to meet the changing loads, which usually vary from very low loading up to as much as 100% of plant capacity, depending on design and operating environment. Multiple chillers also permit designs that incorporate fail safe operation with backup available in case of a failure of one of the machines.
Medium and large size cooling plants often incorporate centrifugal compressors due to their superior operating efficiency and larger capacities. A chiller consists of one or more compressors mounted on a set of heat exchangers which, along with additional piping, refrigerant and other equipment, cools a fluid that flows through one heat exchanger while rejecting the heat absorbed at a higher temperature to a fluid flowing through the second heat exchanger. The fluid flowing through both heat exchangers is usually water. Each set of one or more compressors and two heat exchangers is called a chiller, and medium to large chiller plants consist of multiple chillers.
FIG. 5 is a simplified diagram of components of a conventional water chiller plant with four chillers (501-504) arranged in parallel and connected to chilled water pumping and piping system (520). Each chiller has an individual chiller controller (531-534) each of which is in communication with a controller (540). The individual chiller controllers may communicate via a network such as an Ethernet network. The central controller preferably has a processer and associated memory which are configured to read a computer program. The central controller may be a digital computing system, for example, a personal computer or a programmable logic controller. The central controller is configured to receive information from each individual chiller such as head pressure readings, fluid temperatures and the current vane settings. The central controller also controls various aspects of the chiller plant including pump speed, chiller loading and switching a chiller on or offline.
FIG. 1 illustrates the major components of a variable speed centrifugal chiller. Medium and large chiller plants typically employ from two to as many as a dozen or more such chillers for comfort conditioning applications or to serve process cooling needs in a manufacturing application. In a typical water chiller plant with variable speed centrifugal chillers, each chiller has one or more motor/compressor unit (109), which may be a hermetic type or open type. The motor or engine that drives the compressor is powered by a power unit commonly called a variable speed drive (110) that can vary the rotational speed of the motor or engine in the compressor unit.
Each compressor draws low pressure refrigerant gas from the cooler (111) through a connection (112), compresses it, and discharges it as a higher pressure hot gas through a connection (113) into the condenser (114). In the condenser, hot gaseous refrigerant is condensed into a liquid by rejecting heat to condenser water that is supplied through a piping connection (140) from a cooling tower or some other means of conducting heat from the fluid. The condenser water flows through tubes in the condenser, absorbs heat from the refrigerant and cools it to a high pressure liquid. The heated condenser water then leaves the condenser through a piping connection (141) to return to the cooling tower or other method of heat rejection.
The condensed liquid refrigerant then flows through an expansion device (133) that regulates the flow of refrigerant into the cooler (111), which is held at a low pressure by the operation of the compressor continuously drawing expanded gaseous refrigerant from it. The low pressure environment causes the refrigerant to change state to a gas and as it does so, it absorbs the required heat of vaporization from the chilled water circulating into the cooler via pipe connection (151), then through tubes in the cooler where the boiling refrigerant absorbs heat from the chilled water and the chilled water then exits through a pipe connection (152) at the desired temperature to cool the comfort or process loads to which the chiller plant is connected. The low pressure vapor is drawn into the inlet of the compressor and the cycle is continuously repeated. The chilled and condenser water are typically circulated by pumps not shown. Control of all elements within the chiller is provided by an on-board control panel (162). Though the configuration of many chillers is similar to that shown in FIG. 1, there are variations to this basic design.
FIG. 2 is a cross section that shows in some greater detail the elements of the motor/compressor unit (see 109, 110 in FIG. 1) of a centrifugal compressor used in centrifugal chillers. The compressor unit consists of a motor or engine (210) that rotates a shaft upon which an impeller (212) is mounted that rotates within a housing (214). The compressor inlet (216) is connected to the evaporator (not shown) which may be configured in a number of variations. As the gas to be compressed, which is called “refrigerant,” is drawn into the compressor by the rotation of the impeller, it must first pass through inlet vanes (218) which are segmented. The vanes are closed and opened by coordinated rotation of each segment around its central axis (shown as a vertical axis in FIG. 2). We call this rotational position the current vane position or setting. When closed, only a small hole in the center of the segments is open for gas to pass. When the vanes are set to open, virtually the entire inlet area is open. As the vanes begin to close from full-open; their coordinated movement causes the gas flowing by to be rotated in the direction of the rotation of the compressor impeller (212). This rotational movement of the gas entering the compressor impeller which is rotating in the same direction reduces the flow into and through the impeller. As the vanes continue to close, the vanes further reduce the flow of refrigerant into the compressor inlet by creating a pressure difference across the vanes. The impeller draws the gas in at low pressure and imparts energy to the gas to discharge it at a higher pressure in the volute (220) of the housing (222) where it is collected and routed to the condenser.
Variable speed compressors can reduce their operating capacity in two ways, first, by closing the inlet vanes as described above, and second, by slowing the speed of the compressor impeller. However, impeller rotational speed must always be maintained sufficiently high to maintain the flow of refrigerant gas through the impeller at the current pressure difference between the condenser and evaporator of the chiller. If the speed falls below a minimum speed that depends on this pressure difference across the impeller, the impeller will stall and flow will abruptly stop. The phenomenon in chillers is called “surging.” The impeller stalls and flow stops, this reduces the pressure difference and flow restarts only to stall again. Surging results in inefficient operation and can under some circumstances cause damage to elements of the compressor.
To ensure surging does not develop, the internal chiller or compressor controls of variable speed chillers incorporate some method of maintaining a minimum compressor speed that is usually based on the pressure across the impeller. When operating conditions require a certain pressure differential across the compressor (commonly called compressor “head”) such that the impeller speed cannot be reduced due to a risk of stalling and surging, and at the same time a lower capacity is required from the chiller, then instead of slowing the speed of the impeller to reduce capacity, the impeller is maintained at the appropriate minimum speed and the vanes are closed to reduce the capacity, sacrificing efficiency of the chiller.
There is a need for improvement in operating efficiency of systems of the type described above.